Range finders employing laser radar are being developed for automotive applications such as adaptive cruise control (ACC), an expansion of existing cruise control systems that maintain vehicle speed and separation through a link to the powertrain of a vehicle. A vehicle equipped with ACC usually employs a radar sensor mounted in the front of the vehicle to help maintain a safe cruising distance between itself and a vehicle that it is following. That distance interval, which may be set by an ACC system, guides the acceleration and braking of a vehicle to maintain a proper distance between vehicles. Some systems can compensate for hilly terrain and passing situations.
Currently Lidar systems (Llght Detection And Ranging), i.e. laser range-finding devices, are being used in laser telemetry applications such as ACC to determine the distance of an object from a time-of-flight measurement. Traditional Lidar detection schemes are based on a pulsed or continuous wave laser emitter and a discrete solid-state detector. Transmitting and receiving electromagnetic radiation at a higher frequency than radar, a Lidar system can operate in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. With visible light, the time-of-flight for distances of 50 to 200 meters is typically short (e.g., 167 to 667 nanoseconds to the target vehicle and a similar time for the echo to reach the detector) so that high-speed detectors are needed to capture the signal from the Lidar system.
One approach to velocity and distance detection schemes, described in U.S. Pat. No. 4,743,110 to Arnaud et al., modulates a frequency of an outgoing beam, above and below some reference frequency, and then recombines a Doppler-shifted echo with a reference beam at a photodetector. Another example of current range-finding technology, disclosed in published patent application U.S. 2001/0045981 A1 to Gloger et al., is a method and device for detecting road users and obstacles on the basis of camera images and a distance-measuring sensor such as a radar sensor.
Traditional coherent Lidar detection schemes do not modulate the outgoing laser beam. Since the outgoing beam is not modulated, the beat frequency is typically in the 10's-100's of THz range. This beat frequency can be measured with a high-speed photodiode or photodetector, although it may be too fast to measure with a photoarray of a charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS) camera. Thus, the above-mentioned system with a camera still requires a separate light-sensing detector.
It would be beneficial to have an alternative and simplified range-finding system that could use a low-cost camera, and would not require a separate laser detector. There are difficulties, however, in creating such a system. For example, silicon photodiodes in a CMOS digital camera are good photodetectors in the near infrared and visible light, however, the typical integration time of a camera pixel element may be several orders of magnitude longer than the time-of-flight signal from a laser-ranger finder, an integration time much too slow for conventional Lidar sensing.
It is an object of this invention, therefore, to provide a method, system and device for calculating velocity and distance based on integral laser ranging and velocity measurements from a photodetector imaging array without a separate Lidar detector. The invention will overcome the problem of slow integration time, as well as other the deficiencies and obstacles described above.